I. Field of the Invention
This invention relates to heat-resistant resin compositions and more particularly to heat-resistant resin compositions adapted to be applied, for example, by impregnation, casting, lamination, molding or adhesion.
II. Description of the Prior Art
Electrical insulating material is generally desired to have high heat resistance from the standpoint of rendering electrical apparatuses compact and light and also elevating the safety and reliable performance thereof. A prominently heat-resistant, solventless, impregnation type resin in particular is demanded as electrical insulating material used with D.C. and A.C. motors and transformers.
Such heat-resistant resin known to date is an aromatic polyimide resin. However, this aromatic polyimide resin gives forth water during the hardening reaction. When, therefore, an attempt is made to provide a dense electrical insulating structure from the aromatic polyimide resin, the manufacturing conditions unavoidably become complicated. For this reason, the aromatic polyimide resin has hitherto been applied only as insulating covering or film for electric wires. Further, a widely accepted heat-resistant resin includes an epoxy resin derived from phenol novolak or cresol novolak. When, however, used long in an atmosphere or at a site in which a higher temperature than 150.degree. C always prevails, the epoxy resin noticeably decreases in mechanical strength and electrical insulation, and has proved inadequate to be applied as electrical insulating material under such condition.
Another known heat-resistant resins are those mainly composed of a maleimide compound. When hardened, however, this maleimide resin becomes too brittle for practical application. To compensate for this drawback, customary practice has been to modify the maleimide resin by adding an additive. Such modifications already proposed to this end include, for example, those described in the Japanese Patent Application Publication 11,359/73 (bisimide + epoxy resin + carboxylic anhydride) and the following Japanese Patent Application Disclosures
1745/72 (bismaleimide + diamine + vinyl monomer) PA1 32,944/74 (maleimide + diallylisophthalate) PA1 47,487/74 (bismaleimide + allylamine + carboxylic acid allyl ester) PA1 121,899/74 (bismaleimide + epoxy resin + amide acid imide) PA1 2099/75 (bisimide + epoxy resin + monoimide + polyamine) PA1 13,268/75 (bisimide + epoxy resin + polyamine) PA1 13,497/75 (maleimide + epoxy resin + vinyl compound + metal chelate compound) PA1 21,098/75 (polymaleimide + epoxy resin + hardening agent) PA1 76,194/75 (maleimide + epoxy resin + diallylisophthalate polymer) PA1 96,696/75 and 96,697/75 (polyimide + polyhydric phenol + catalyst)
However, any of the proposed compositions has the drawbacks that the maleimide compound has a relatively low compatibility with the above-listed additives and tends to give rise to sediments in the proximity of room temperature. Consequently it is necessary to restrict the content of the compound to less than 30% by weight on the basis of the total amount of the composition, and the resultant composition fails to show sufficient heat resistance. Further the Japanese Patent Application Disclosure 994/77 teaches a heat-resistant composition formed of maleimide compound, and alkenyl phenol and/or alkenyl phenol ether, which has low initial electric properties when hardened at a lower temperature than 200.degree. C and lacks affinity to inorganic material such as glass cloth.